While current mobile phone calls offer much needed flexibility for communicating no matter with respect to locations, oftentimes mobile phone calls between two people occur while the called party is in a public environment.
There are situations where the caller (i.e., Calling Party) only wants to talk with the receiver (i.e., Called Party) and without the risk of being exposed (such as in a conference with other parties) or audibly heard by another third party who happens to be in the vicinity of the called party whether accidentally or deliberately.